This has not at all been the case for me. LLMs (even the more-capable local models) are exceedingly good at troubleshooting when given proper context (clues, hints, a decent system prompt/skill that provides even the barest hint of how things work)

What you're describing sounds more like what would happen if you gave a chatbot without access to the repo as little context as possible and asked it to come up with a solution, blind.